Metal surfaces, such as those of steel and/or aluminum and their alloys, frequently contain unattractive and corrosion-promoting oxide layers which may be formed as a result of environmental exposure and/or heat treating operations, such as welding. Typically, such oxide layers are removed by treating the metal surfaces with an acid solution, such as those based on hydrochloric, sulfuric, phosphoric or hydrofluoric acid, or by treatment with an acid-containing paste. The latter compositions are normally applied only to the areas which are to be cleaned, and are frequently used on large articles or where there is only localized oxide formation. As a result of the treatment with the acid-containing solutions or paste, a part of the oxides and the metal substrates adhering to the oxides are dissolved so that the oxides lose their adhesion to the metal surface. After an appropriate contact time, the solutions or paste are rinsed off with water or removed with brushes.
Exemplary of such acid cleaning and etching solutions is that disclosed in German Patent No. 1950560, which contains hydrofluoric acid, a magnesium compound, at least one acid selected from nitric acid, phosphoric acid and sulfuric acid, either in the form of free acid or the acid salts, and a particular sulfonic acid. Exemplary of pickling or derusting pastes which have been used are those disclosed in German Patent No. 1082475, which contain phosphoric acid and a thickener, such as a hydrolyzable oil or fatty acid which can be converted to a soap. A further pickling and derusting paste which has been proposed is described in U.S. patent application Ser. No. 346331, now U.S. Pat. No. 4,400,289 filed Feb. 5, 1982 (German Published patent application No. 3105508). This composition contains fluoride ions, a mixture of compounds of calcium and trivalent iron and/or aluminum and has an acidity corresponding to at least 10% by weight of free acid, calculated as 100% HF. This pickling paste composition is particularly suitable for the treatment of steel alloys.
The use of pickling pastes, particularly those which contain fluoride, frequently present problems which are not encountered when pickling solutions are used. For example, where the pickling pastes are used on relatively large articles, suitable rinsing tanks may not be available. Additionally, the waste waters formed when the spent pickling pastes are rinsed from metal surfaces, which waste waters are strongly acidic and contain fluoride, may not be discharged to municipal sewer systems or otherwise discarded, without being treated to reduce the acidity and remove the contained fluoride ions. Typically, it is customary to collect such waste waters and treat them with calcium hydroxide to raise the pH to within the range of about 6.5 to 9.5. The calcium hydroxide further reacts with the fluoride ions in the waste waters to form insoluble calcium fluoride which are separated from the solution and the remaining liquid, at a neutral or slightly alkaline pH, can then be discharged to municipal sewage systems.
This procedure, however, presents some difficulties in that the solutions or suspensions of calcium hydroxide, or milk of lime, do not completely react immediately with the acidic, fluoride-containing waste waters. Thus, although the amount of calcium hydroxide added initially results in a solution pH within the desired range, as subsequent reaction occurs, the solution pH rises above the permissible level. Accordingly, in treating these waste waters in this manner, precise checking of the solution alkalinity and frequent readjustments of the pH are required, which involves repeated and time consuming monitoring steps.
To overcome this difficulty, it has been proposed that the fluoride-containing acidic waste waters can be treated by flowing them over granulated calcium carbonate, e.g., marble. This procedure, however, has been found to have little commercial application since the calcium carbonate rapidly becomes coated with insoluble calcium fluoride, thus preventing any further reaction from taking place.
It is, therefore, an object of the present invention to provide an improved process for the treatment of spent pickling paste on metal surfaces.
A further object of the present invention is to provide an improved process for the treatment of spent pickling pastes to convert them to a form in which they may be safely disposed of, which process overcomes the disadvantages of the prior art processes and is simple and economical to carry out.
These and other objects will become apparent to those of ordinary skill in the art from the description of the invention which follows.